Voluntary movement is the result of the interactions of a limb and the environment. Forces generated by the muscles spanning the limb depend on both muscle activation and external load due to intrinsic muscle visco- elasticity. To generate a movement with a desired trajectory the muscles should generate appropriate forces which means that muscle activation accounts for an expected load. In cases when the actual movement trajectory deviates from a desired trajectory muscle activation may be modified by proprioceptive feedback. The primary goal of this proposal is to determine how the central control and proprioceptive feedback interact during fast movement. The time course of the central control depends on a movement task and varies with movement distance and/or expected load. Unexpected changes in the load alter the movement trajectory and evoke corrections in muscle activation due to proprioceptive feedback. The proposed experiments will focus on the relationship between the movement task and the timing of changes in muscle activation. Subjects will perform movements over different distances against light and heavy inertial loads. For every movement task, in a small subset of trials, the experimenter will unexpectedly change the load. The patterns of surface electromyograms (EMG) and trajectories of movements made against an expected load will be compared with the corresponding traces during movements in which the load was changed. We will systematically manipulate the movement distance and expected load, as well as the parameters of the unexpected load. These experiments will elucidate the underlying mechanisms which influence proprioceptive feedback during fast voluntary movements.